Fluid energy mill



June 1, 1965 R. M. MANDLE ETAL 3,186,648

FLUID ENERGY MILL Filed May 27, 1963 yls' Zfl

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INVENTORS RICHARD M. MANDLE THOMAS O. TONGUE ATTORNEY United States Patent 3,186,648 FLUID ENERGY MILL Richard M. Maudie, Pompton Lakes, N.J., and Thomas 0. Tongue, Baltimore, Md., assignors to W. R. Grace & Co., New York, N.Y., a corporation OfCOEHlQCilCUt Filed May 27, 1963, Ser. No. 283,459

1 Claim. (Cl. 241-34) This invention relates to improvements in fluid energy mill operations. In one specific aspect, it relates to the elimination of static electricity in fluid energy mills.

Particle size reduction of ores and other hard materials is an important industrial process. One of the many techniques that can be used to accomplish this reduction is attrition grinding as obtained by the use of the fluid energy mill.

In the operation of a fluid energy mill, gas of a high energy content is introduced into a pulverizing chamber. Feed materials in the chamber are caused to impinge upon themselves at high velocities while entranied in the gas stream, causing a reduction in particle size. Twocommon types of fluid energy mills are the Micronizer and the jet pulverizer. Depending upon plant conditions and the material to be processed, these mills are operated by steam, gas or compressed air.

One of the problems encountered with the use of compressed air is the generation of static electricity. The

problem is especially aggravated when the air is heated.

to elevated temperatures, causing a hot, dry atmosphere in the mill. In this atmosphere, the impingement of the particles upon themselves generates a static charge. This charge presents two problems, (a) the flow of ground and semi-ground particles is impeded, thus reducing the capacity and general performance of the mill, and (b) material recovered in the collection bags is diflicult or impossible to remove by the normal blow back procedures. This causes a back pressure to develop which in turn upsets overall balance and the mill has to be stopped.

This problem is particularly troublesome in the grinding of the mineral bastnasite to make glass polishing powder.

Heretofore, there has been, no known method for preventing the formation of static electricity in the fluid energy mills.

It is, therefore, an object of this invention to provide a method for preventing the accumulation of static electricity in a fluid energy mill. 7

It is also an object to provide a method of grinding dry solids.

These and other objects of the present invention will become readily apparent from the following detailed description, specific examples, and drawing wherein;

FIG. 1 is a schematic diagram of a typical fluid energy mill modified in accordance with the present invention; and

FIG. 2 is a horizontal sectional view of the apparatus of FIG. 1 taken along line A-A of FIG. 1.

Broadly, the present invention contemplates the introduction of water into a fluid energy millto reduce the static charge on the ground particles.

A typical fluid energy mill used in the practice of this invention is a jet pulverizer, although it should be understood that the detailed description and specific examples, which indicate preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications within the spirit and scope of the invention opposing jets 13, through which high velocity heated air "ice is fed. The material 14 between the jets 13 is maintained in a high state of turbulence and is pulverized. The pulverized particles are swept from the grinding zone 12 by an auxiliary current of room air through a conduit 15 into a classifier 17, the line particles remain suspended in the air and are swept from the classifier 17 (such as a cyclone) through conduit 18 into a filter 1 9 (suchas a collector). The coarser particles from classifier 17 are passed through conduits 20 to hopper 21 where they are picked up by Venturi tubes 23 and returned to the grinding zone 12 through jets 13 for further processing. The high energy gas for grinding the particles enters the mill through conduit 24 preheated by heat exchanger 25. The preferred gas for this invention is. compressed air at elevated temperatures. 7

Water as an atomized spray or as steam may be introduced into the grinding zone 12, through a separate nozzle 26 or by injection into the compressed air stream 24. If desired, it may bev introduced at both points simultaneously. In the injection of water or steam into the pulverizing chamber 12, it is advantageous that the atomized spray 27 be directed into the mass of commuting particles 14. A spray nozzle 25 is preferably utilized to inject the water or steam in the grinding zone 12 to avoid excessive localized wetting, which may cause agglomeration of particles.

'The amount of water necessary to eliminate the static charge is obviously dependent upon the moisture content of the material to be ground and on the moisture content of the compressed air. The moisture in the air will vary with the outside atmospheric conditions. For example, on dry days the moisture content will be less than that of rainy days. It is desirable that the temperature of the efliuent air 29 be maintained at about 50 F. above the dew point temperature to facilitate the collection of the fine particles in filter 19.

Various methods are available for determining optimum conditions for continuous mill operations. Among them are the determination of the relative humidity of the efliuent air 29, continuous moisture analysis of the efiiuent air 29, and a determination of the electro-static charge on the particles.

We have found that the amount of water or steam injected into the grinding zone 12 can be controlled by inserting a probe 30 into conduit 18 between the classifier 17 and the bag collector 19, and connecting the probe 30 to an electro-static or vacuum tube voltmeter 31 such as a Farranti or R.C.A. vacuum tube voltmeter for static high voltage determination. The voltmeter in turn is connected to a recorder-controller 32 such as a Minneapolis Honeywell Electronic #15 Round Chart Recording-Controller; this device controls valve 33 monitoring the water or steam flow. The valve 33 may be a Minneapolis Honeywell 904E valve positioner, balancing relay t ype. In operation a signal from probe 30, transmitted through the voltmeter 31, to the recorder-controller 32 actuates valve 33 to emit more or less water or steam to the grinding zone 12 as needed. The recorder-controller 32 may 1 be by-passed for manual operation of valve 33, if desired.

This invention is applicable to the size reduction of a wide variety of materials capable of generating a static charge in a fluid energy mill. Examples of these materials are aluminum oxide, ceramic frit, powdered insecticides such as DDT, diatomaceous earth, feldspar, fluorspar, graphite, gypsum, iron ore, iron oxide, iron powder, linestone, mica, paint pigments, polymers, rare earth ores carbon, talc, and the like.

The reduction of static electricity in a jetpulverizer isshown in the following examples.

'the opposing air jets.,

. 3 EXAMPLE 1 The mill used in this example was a Majac jet pulverizer of the type described elsewhere in the specification. This mill was equipped with an oilburner nozzle 26 that sprayed a fan shaped mist 27 of water directly between Test 1.Bastnasite ore was fed ata rate of 300 pounds per hour into the jet pulverizer under. the following conditions: grind aha-800 cubic feet per'minute at 100' pounds per square inch at 750 F. "Sweep air-4000 cubic feetper minute at 1 to 2 pounds per square inch at ambient temperature. Was inserted in the line between the classifier 17 and the bag collector 19 and connected to a vacuum tube voltmeter 31 to measure the extentof electrostatic charge in theground particles. The charge registered was in excess of 1000 volts, and within 2 hours of this reading the mill An electrostaticsensing probe 30 7 was shut down due to the excessive pressure drop across the bag collector. g V a Test 2.-Bastnasite ore was ground under the same conditions asin' Test 1, except that the control valve 32 was operated manually and water was injectedthrough nozzle 26'into the pulverizing chamber 12 ata rate of 9 gallons per hour. The static charge of the groundparticle s was reduced to 500. volts and, the freely through the mill.

bastnasite ore flowed,

Test 3.This test was a repeat of Test 2, with the water injection rate raised to 15 gallons per hour. The charge on the ground particles as measured on the volt meter was 200 volts. The bastnasite flowed freely through the mill,c0uld be removed readilyfrom the bagcollector and the grinding capacity of the mill was increased.

by 25 percent.

dry heated gasstream and separation of comminuted particles from said gasstream is impeded by the buildup of static electric charges on said particles, means for injecting water into the particle stream passing through said mill .to dissipate the static charge on said particles, means for monitoring static electric charge on the particlesiof said stream, water controlling means w for" controlling the amount of Water injected 'by said injection means, said controlling means responsive 'to' said monitoringmeans whereby the amount of water injected into said particle stream is proportional to the amountrequired to substantially dissipatethe static electric charge thereon as, determined by said monitoring means. 7

V Reierences Cited by the Examiner y NITED STATES PATENTS 7 238,044 2/81 Luckenbach et al. 241-5 1,058,31-3 14/ 13 Luckenbach V 2415 2,412,586 12/46 Knowland.j r 241-7-15X 2,833,482. 5/58 Weston et a1 241-15 X 2,885,154 v5/59 Eastman et al 241.-5 2,914,391 11/59 Stratford. 241-5X 3,168,253 2/ 5, ,Masuda 241- 16 YFOREIGNIPATVENTS 986,566 3/51 Fra'nce.

J J. SPENCER -OVERHOLSER,*Primg1-y Examiner.

WILLIAMW.DYER,1JR., Examiner. 

